This does not seem like an issue with IIRF.
It seems like your Firefox user/browser is simply not passing HTTP_REFERER.

There are various plugins for Firefox that can strip HTTP_REFERER. is it possible that one such is enabled and working? If it is, it may be the culprit.

http://www.google.com/search?hl=en&num=30&q=firefoxpluginreferer

Some other things to try, to learn more about your scenario:
point firefox at a server-side script that is not fronted by IIRF; does HTTP_REFERER get passed?
examine any firewall/proxy settings on the browser side that may obscure or modify HTTP_REFERER